Pulsed fiber lasers are currently of great interest for a variety of applications. One such application is scribing semiconductor materials. Material ablation with laser pulses can be separated in two distinct regimes of operation; thermal and non-thermal. In the thermal regime, the laser energy is transferred to the material lattice through electron-phonon interactions. If atoms are ejected from the lattice before such interactions can really take place, than the process is considered non-thermal. The timescale over which the energy transferred to the electrons by the laser pulse is further transferred to the lattice is of the order of tens of picoseconds (typically 5-50 ps depending on materials). Consequently, picosecond pulses with a duration greater than this characteristic timescale are considered the shortest pulses that can still be considered to operate in the thermal regime.
Advantageously, micro-machining with the shortest pulse in the thermal regime reduces to the minimum the size of the heat affected zone (HAZ) surrounding the targeted region. This is highly relevant in applications where multiple layers are stacked and only one of those layers is targeted, such as for example in the drilling of via in photovoltaic cells used in solar panels.
Picosecond pulses are characterized by high peak power (ten to hundreds of kilowatts for micro joules pulses) and narrow linewidth (less than 1 nm for transform limited pulses). This combination is very advantageous for frequency conversion (second, third and forth harmonic), which opens up significantly the range of applications a single powerful picosecond source can address.
Mode-locked femtosecond laser, bulk or fiber-based, can be modified to produce picosecond pulses. Generally speaking, mode-locked fiber lasers are considered particularly attractive structures for ultra-short pulse generation, via either passive or active mode-locking. The pulse-generation mechanism in such lasers depends on the physics of the cavity. Known cavity configurations include linear cavities, ring lasers and figure-of-eight cavities. To produce picosecond pulses in such a mode-locked regime, a narrow spectral filter placed inside the laser cavity controls the duration of the pulses by the virtue of the Fourier transform. Those designs are usually not very flexible since they necessitate a tuning of the filter bandwidth to change the pulse duration. This tuning can necessitate moving parts.
Picosecond pulses can also be produced with gain-switched semiconductor diode lasers, where the pulses are advantageously generated on demand by an electrical pulse. However there is little correlation between the electrical pulse sent and the received optical pulse. The optical pulse is in fact the impulse response of the device, and therefore has a duration which differs from chip to chip. In addition, such diodes offer very little control on the spectral content of the emitted pulses, which is usually quite broad, and the optical pulse is often followed by relaxation oscillations.
There remains a need in the field for picosecond fiber lasers suited to the requirements of micromachining and similar industrial applications.